The nature of resistance to Neonectria ditissima in apple species

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In the UK, land devoted to modern apple production is rising at a rate of between 7 and 10% per annum, however (similar to the global situation) the combination of more intensive planting of canker-susceptible cultivars has led to an increase in losses due to the fungal pathogen Neonectria ditissima. Problems in both in-field production (tree death and yield reduction) and post-harvest losses (due to postharvest rot), call for an innovative approach to combat this disease.Approximately 50% of apples produced in the UK are cultivars derived from two progenitor apple varieties, Cox and Gala, both of which are susceptible to fungal canker, indeed these newer varieties such as Kanzi, Rubens and Jazz are extremely susceptible to canker, which has led to large problems in orchard establishment and increasingly post-harvest losses, especially in wet years. In severe cases, in field yield decline can reduce potential pre-harvest yield by at least 25% due to loss of fruiting wood and 5% of postharvest apple crops are lost due to Neonectria infection. There is poor chemical control of canker of trees due to its systemic nature of the disease. It has been shown that the pathogen is often present in the plants from a young age (in the nursery) and that both the rootstock and the scion can be infected, but remain asymptomatic for long periods of time. Natural resistance to canker has been well documented, but extremely poorly deployed in modern rootstock or scion breeding programmes. The reasons for this can be attributed to the complex nature of resistance (making selection without molecular markers difficult), the cost of mass screening to eliminate highly susceptible individuals, different types of resistance (all of which are uncharacterised), the relatively recent re-emergence of the problem and the potentially heterogeneous nature of the pathogen. This proposal will use cutting-edge genomics approaches to identify natural plant-derived resistance to this important pathogen in both scion varieties and in rootstocks. In this project, a multi-parental population will be used to characterise resistance and a combination of targeted genome sequencing techniques and pedigree-based genotyping will be used to accurately narrow down the specific genes responsible for resistance. Once obtained, in proof of principle experiments, genes underpinning the resistance will be transferred into a susceptible cultivar, in order to validate the resistance in other genetic backgrounds. The industry are supporting this proposal through the LINK scheme, as they recognise the need for a comprehensive strategy to identify the genetic basis of disease resistance, for use in marker assisted breeding (MAB) and other breeding techniques.